THE EARTHWORM 295 



Excretory System. — The excretory organs of the earthworm 

 consist of paired tubules, called nephridia (Figs. 142 and 143 D), 

 and located in every segment of the body with the exception of the 

 most anterior region. At one end, each nephridial tubule opens 

 from the ccelome by a funnel-like nephrostome. At the other, 

 it opens on the surface of the body by a nephridiopore. The 

 nephrostome is a cilated structure with lip-like edges. It pro- 

 jects anteriorly from the face of the septum on either side, in the 

 ventral region near the nerve cord. From this point the nephrid- 

 ium extends posteriorly through the septum to the next com- 

 partment of the ccelome, where it is folded upon itself in the 

 parallel loops that compose the principal bulk of the organ. 

 The terminal portion is enlarged as a bladder-like structure. 

 The loops of the tubule, as thus formed, are held in place by 

 connective tissue attached to the posterior surface of the 

 septum, and the whole is covered with peritoneum. The tuliule 

 is ciliated for a short distance back of the nephrostome and again 

 in one of the loops. Except for the terminal enlargement, the 

 nephridial tubule is an intracellular duct. The terminal enlarge- 

 ment is lined with a non-stratified squamous epithelium, outside 

 of which are muscle fibers. Each nephridium is well supplied 

 with blood vessels, so that the relationship of the nephridium 

 and the blood system resembles that of a single kidney tubule 

 and its capillaries in a vetebrate animal (cf. Fig. 31, p. 53). 



Reproductive System. — The earthworm is hermaphroditic, each 

 individual containing a complete set of male and female reproduc- 

 tive organs. The female system (Fig. 143 D) consists of a pair of 

 ovaries attached to the posterior face of the septum between 

 segments twelve and thirteen and projecting backward into seg- 

 ment thirteen. Upon microscopic examination, the ovary (Fig. 

 143 A) is seen to contain at its posterior end ripe ova, which will 

 soon be detached and passed into the oviduct; while anteriorly 

 it contains eggs that are in earlier stages. Immediately behind 

 the posterior end of each ovary is the oviduct, which is a short 

 tube with a ciliated funnel opening from segment thirteen and 

 thence leading through the septum to segment fourteen, where 

 it passes diagonally outward to its external opening. A pocket- 

 like outgrowth on each oviduct just posterior to the septum, 

 functions as a receptacle in which the ova that have been detached 

 from the ovary may be temporarily stored. The seminal recep- 



